For example, as a developing unit, JP-A-9-319202 discloses a related developing device, in which a developer cartridge accommodating developer is detachably attached to a frame provided with a developing part having a developing roller. In the related art developing device, a hole formed in one side portion of the developer cartridge and a hole formed in the frame form a developer supply opening, and a hole formed in the other side portion of the developer cartridge and a hole formed in the frame form a developer receiving opening.
The developer in the developer cartridge is supplied to the developing part through the developer supply opening, but a part of the developer is returned to the developer cartridge through the developer receiving opening. Accordingly, the developer is circulated between the developing part and the developer cartridge.